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a continuous-operation machine for moving bulk, lump, or piece materials.
Historical survey. Chain pumps, which may be regarded as the prototypes of present-day flight conveyors, were used in ancient China and India several millennia before the Common Era for the continuous supply of water from bodies of water to irrigation systems. Bucket and screw water lifters, the forerunners of modern bucket elevators and screw conveyors, were used in Mesopotamia and ancient Egypt. The first attempt to use flight and screw conveyors to move bulk materials (for example, in flour-milling) came in the 16th and 17th centuries. In the late 18th century conveyors began to be used regularly to transport light bulk materials for short distances. In the 1830’s conveyors with belts of strong fabric were introduced for the same purpose. In the second half of the 19th century conveyors began to be used in industry to deliver heavy bulk and piece materials. Expanded application of conveyors brought about the development and introduction of new types of conveyors: belt conveyors with belts of rubberized fabric (1868, Great Britain), fixed and movable apron conveyors (1870, Russia), screw conveyors with spiral screws for lump materials (1887, USA), pivoted bucket conveyors with hinge-mounted buckets for delivering goods over complex routes (1896, USA), belt conveyors with steel belts (1905, Sweden), and inertial conveyors (1906, Great Britain and Germany). In 1882 a conveyor was used to connect production units in mass flow production. Cast floor-mounted conveyors (1890, USA), suspension conveyors (1894, Great Britain), and special assembly conveyors (1912–14, USA) began to be used somewhat later.
In the 1880’s the manufacture of conveyors in the industrially developed countries gradually became a separate area of machine building. Modern conveyors have retained their main design elements, which have been refined in accordance with the advances of science and technology (the substitution of electric drive for belt drive and the use of vibration engineering and the energy of compressed air).
Classification. The main characteristics of conveyors for purposes of classification are the type of traction and the load-carrying elements. A distinction is made between conveyors with belt, chain, and cable drive elements and conveyors without a drive element (gravitational, inertial, and screw conveyors). Conveyors with drive elements can be differentiated by type of load-carrying elements into belt, apron, cradle, flight, and bucket types. Such conveyors are characterized by movement of the load together with the working element. The thrust is transmitted either by the load-carrying element or by an element that pushes or pulls the material along a fixed trough, tube, or bed. In conveyors without a drive element the movement of the load is usually separate from the movement of the working elements, which revolve in roller and screw conveyors or perform reciprocating motion in, for example, inertial conveyors. Conveyors may have a machine drive (most often electric; sometimes pneumatic), or the load may be moved by the force of gravity (gravitational conveyors).
The choice of floor or suspension conveyors depends on conditions. Floor conveyors may be fixed or movable. Loads on a conveyor may be moved horizontally or nearly horizontally (belt, apron, car, flight, roller, screw, shaking, and pendulum conveyors), vertically or nearly vertically (flight, bucket, screw, and shaking conveyors), or in more than one plane. In the last case the conveyor consists of successive horizontal, vertical, or inclined segments (suspension, bucket, flight, cradle, and other types). In addition, conveyors may be differentiated according to the type of materials for which they are used (loose or piece cargo). The design of some conveyors makes it possible for them to move either loose or piece materials. Special groups of conveyors are constituted by elevators, vertical conveyors with suspended buckets, cradles, or shelves; escalators, special apron and belt conveyors for moving people, walking conveyors, trimmers, stackers for stacking logs, and combined types (for example, the Rapistan roller-belt conveyor, which ensures that piece cargo is kept at designated intervals on descending routes).
Main types. Belt conveyors are used to move bulk and piece cargo over distances up to 10–12 km in some cases (see Figure 1). Such conveyors are usually made up of separate sections. The route of the conveyor in the horizontal plane is straight; in the vertical plane it may be inclined or have a more complex configuration. The traction and load-carrying element is the belt, which moves on fixed roller supports and goes around drive, tensioning, and sometimes deflecting drums. The cargo moves with the belt. The belt may be flat or grooved, depending on the type of roller supports. Conveyors with flat belts are used primarily for moving piece goods. The tensioning station provides the required belt tension. It is usually a load station; in mobile conveyors it is a screw station. The conveyor drive (drive station) consists of an electric motor, a reduction gear, a drum, and couplings. Bulk cargo is loaded onto the belt through a guide chute or funnel and unloaded by an end drum or by a plow or drum kicker. Belt conveyors have high reliability and throughput levels from several tons to several thousand tons per hour. The fabric belts in such conveyors are 300–2,000 mm wide, and the belt moves at speeds of 1.5˗4.0 m/sec. Short mobile belt conveyors are mounted on wheels and used for loading and unloading and in construction.
Apron conveyors are used to move heavy piece cargo (500 kg and more), large pieces (including sharp-edged materials), and cargo heated to high temperatures in the horizontal plane or at small inclinations of up to 35° (see Figure 2). Fixed and mobile apron conveyors have the same basic elements as belt conveyors. The load-carrying element is a metal or, less frequently, wood or plastic bed consisting of separate slats mounted on one or two sleeve-roller drive chains. The bed may be flat or wavy or have a rectangular cross section, with or without sides. The drive chains pass around the drive and tension sprockets, which are mounted on the ends of the frame. A distinction is made between general-purpose slat conveyors (the main type) and special conveyors. The latter includes routed conveyors, pouring machines for metal, and passenger escalators. The load moves slowly (0.3–1.0 m/sec). Fixed sides are added to conveyors with flat beds to increase their output. Standard apron conveyors move up to 2,000 tons per hour.
Flight conveyors transport cargo by flights (or scrapers) that move along a trough or tube. Such conveyors are used for processing bulk or piece cargo, which enters the trough through a loading funnel. The working arm is ordinarily the lower one; less often it is the upper, and sometimes it is both. The cross section of the trough and the configuration of the flights must be identical (rectangular, semicircular, or trapezoidal). The flights are stamped from sheet steel or cast, and the troughs are ordinarily of metal and sometimes wood. Flight conveyors weigh less than apron conveyors and can be loaded and unloaded at any point along the trough. The use of flight conveyors is limited because of pulverization of the cargo and rapid wear of the trough, particularly when moving abrasive materials. In addition, flight conveyors usually consume a great deal of energy in overcoming negative resistances. The speed of the working element of flight conveyors is 0.16–0.5 m/sec (less frequently 1.0), and the productivity is 50–350 tons per hour. Flight conveyors are usually used for moving cargo over distances of up to 100 m.
A variation of the flight conveyor is the conveyor with submerged flights, in which the flights cover only part of the cross section of the trough and the load fills the entire working arm of the trough or a large part of it. Such conveyors can have complex routes and are used for moving cargo (usually fine bulk goods) horizontally, vertically, and on an incline at speeds of 0.1–0.25 m/sec. Tube conveyors, in which the drive chain and flights are inside the tube and the flights fill the entire cross section of the tube, constitute a special group of flight conveyors. Such conveyors can also have a complex path.
Unlike other types of conveyors, conveyors with drive and carrying chains do not have a load-carrying element and are used primarily in flow lines during conveyor assembly. On conveyors with carrying chains the cargo is placed directly on the drive chains, which slide in fixed guides. On conveyors with drive chains the cargo moves along fixed routes on the floor of the shop or has its own wheeled or caterpillar running gear. Car conveyors are used in assembly work in mass and series production. They are cars connected by a drive chain that move along a closed route. The main processes of casting production (molding, teeming, and cooling) and the assembly of machines are done on the cars.
Suspension conveyors with chain drive elements are used for continuous (less frequently, periodic) movement of piece cargo. The routes of such conveyors are usually closed and have complex shapes. Suspension conveyors are divided into three groups: load-carrying conveyors (the load carriers are permanently connected to the drive elements), pulling conveyors (the carriers are also permanently connected to the drive element and have hooks for attaching cars, which move along the floor of the shop or warehouse), and pushing conveyors (the carriers are not permanently connected to the drive element and move along suspended routes). The use of suspension conveyors makes possible full mechanization and automation of loading-unloading and warehouse work at the junctions of intrashop, intraplant, and mainline transportation. They also play a significant role in the creation of fully automated warehouses.
Screw conveyors are used to move powdered and fine-grained materials in the horizontal plane or on an incline up to 20 percent (see Figure 3); less frequently they are used on the vertical plane (conveyors with rapidly turning screws). Such conveyors have an enclosed metal trough inside which revolves a shaft with blades mounted with a pitch. The blades may be solid for light bulk cargo, bandlike for moist and piece cargo, or in the form of vanes individually fastened to the shaft for sticky and caking cargo. When the screws turn, the blades propel the cargo along the trough. Screw conveyors consist of sections 2–4 m long, with the total length not usually more than 60 m. The diameter of the trough is 100–600 mm. Screw conveyors are simple in design and convenient in operation, especially for transporting powdery cargo. However, the blades and troughs of the conveyor wear out comparatively quickly, and the cargo is ground up and pulverized; in addition, the mechanism consumes a great deal of energy.
Roller conveyors are used to move piece goods with a flat, corrugated, or cylindrical surface (see Figure 4). The rollers turn on bearings on fixed axles of the conveyor frame. The length of the roller should be slightly greater than the width or diameter of the cargo, and the distance between rollers is somewhat less than half the length of the cargo. Small goods of complex shape are moved on such conveyors in boxes or on pallets. There are two types of roller conveyors: gravitational and driven conveyors. In gravitational conveyors, which are installed with an inclination of 2°-5°, the rollers turn freely because of the force of gravity of the cargo being moved. In driven conveyors the rollers are driven as a group by an engine. Such conveyors are used when the cargo must be moved at a constant speed or in a strictly horizontal plane, or when it must be raised at a certain angle. Roller conveyors consist of sections 2–3 m long. Depending on the configuration the route of the conveyor may include curving and hinged sections, turning circles, and switch crossings.
Inertial conveyors are used to transport bulk cargo (less frequently, piece cargo) over comparatively short distances horizontally or on an incline (up to 20°). In inertial conveyors the particles of cargo slide along the load-carrying elements or fly through the air because of inertia. Inertial conveyors are divided into two groups: shaking conveyors, which are characterized by high amplitude and low frequency of oscillation, and vibration conveyors, which have low amplitude and high frequency of oscillation.
In the simplest type of shaking conveyor the trough is located on flexible supports that are rigidly secured to a support frame at a certain angle to the vertical plane (see Figure 5). A crank mechanism driven by an electric motor communicates shifts in direction of movement to the trough. When the trough moves forward it rises slightly, and when it moves back it drops (shakes). When this happens the pressure of the cargo on the trough changes. When the trough moves backward the cargo slides forward along it, moving a certain distance.
On a vibration conveyor asymmetrical oscillations are imparted to the cargo. As a result of the smooth movement of the conveyor tube upward and a sharp movement downward, the particles of the cargo are pulled away from the surface of the pipe and moved along it. Depending on the diameter of the trough (350, 500, or 750 mm), the throughput capacity of the vibration conveyor will be 50, 75, or 150 tons per hour, respectively. The highest possible throughput capacity is 400 tons per hour, and the greatest length is 100 m. Special types of vibration conveyors are also used to move cargo upward.
Technical and economic characteristics. The efficient use of conveyors in any production process depends on the extent to which the type and parameters of the conveyor fit the characteristics of the load and the conditions under which the production process takes place. Among the conditions are throughput capacity; length of conveyance; shape of the route and direction of movement (horizontal, inclined, vertical, and combined); conditions of loading and unloading; the dimensions, shape, specific density, abrasiveness, piece size, moistness, and temperature of the cargo; the rhythm and intensity of the feed; and various local factors.
The throughput capacity Q of any conveyor when moving piece cargo weighing G kg at a speed of v m/sec is determined according to the formula
where a is the distance (in meters) between loads on the conveyor. When bulk or liquid cargo is being moved the formula is
where Ψ is the coefficient for the degree to which the cargo fills the container, I is the volume (in liters) of the container in which the cargo is moving, γ is the density (in tons/m3), v is the speed (in m/sec), and a is the distance (in meters) between containers on the conveyor. When bulk cargo is being moved in a continuous flow, the formula is
Q = 3.6q ˗ v
Q = 3.600F ˗ v tons/hr
where q is the specific load (in kg/m), F is the cross section of the flow (in m2), and v is the speed (in m/sec).
The expressions shown above reveal that both the throughput capacity of a conveyor and the parameters that define it (v, a, F, and so on) do not depend on the distance over which the cargo is moved. This is the principal advantage of continuous-operation machines, to which class conveyors belong, over cyclical-action machines (for example, hoisting cranes, trucks, and trolleys).
The degree of technical advancement of conveyors is characterized by the specific power consumption KN:
where N4 is the capacity (in kilowatts) and Q is the output in tons per hour.
High throughput capacity, simplicity of design and comparatively low cost, the possibility of performing various production operations, the low labor-intensiveness of the work, the provision of labor safety, and improvement in working conditions are factors that have led to the wide use of conveyors in all areas of the national economy: ferrous and nonferrous metallurgy, machine building, mining, chemistry, and the food industry.
Conveyors are an integral part of industrial production. They make possible the establishment and regulation of the rate of production and make the process rhythmic. Although they are the principal means of complete mechanization and automation of transportation and loading-unloading processes and flow production operations, conveyors also free workers from heavy and labor-intensive transportation and loading-unloading jobs and make their labor more productive. Extensive use of conveyors is one typical feature of developed industrial production: the introduction of automatic devices for loading and unloading, metering, counting, weighing, cleaning, and lubrication, as well as various types of monitoring, protective, and interlocking and automatic control equipment, is impossible without using conveyors as one of the basic machines in the automated production system.
REFERENCESZenkov, R. L., and M. M. Petrov. Konveiery bol’shoi moshchnosti. Moscow, 1964.
Spivakovskii, A. O., M. G. Potapov, and M. A. Kotov. Kar’ernyi konveiernyi transport. Moscow, 1965.
Transportiruiushchie i peregruzochnye mashiny dlia kompleksnoi mekhanizatsii pishchevykh proizvodstv. Edited by A. Ia. Sokolov. Moscow, 1964.
Spivakovskii, A. O., and V. N. D’iachkov. Transportiruiushchie mashiny, 2nd ed. Moscow, 1968.
E. I. RIDEL’
A horizontal, inclined, declined, or vertical machine for moving or transporting bulk materials, packages, or objects in a path predetermined by the design of the device and having points of loading and discharge fixed or selective. Included in this category are skip hoist and vertical reciprocating and inclined reciprocating conveyors; but in the strictest sense this category does not include those devices known as industrial trucks, tractors and trailers, cranes, hoists, monorail cranes, power and hand shovels or scoops, bucket drag lines, platform elevators, or highway or rail vehicles. See Bulk-handling machines
Gravity conveyors provide the most economical means for lowering articles and materials. Chutes depend upon sliding friction to control the rate of descent; wheel and roller conveyors use rolling friction for this purpose.
Gravity chutes may be made straight or curved and are fabricated from sheet metal or wood, the latter being sometimes covered with canvas to prevent slivering. The bed of the chute can be shaped to accommodate the products to be handled. In spiral chutes centrifugal force is the second controlling factor. Spirals with roller beds or wheels provide smooth descent of an article and tend to maintain the position of the article in its original starting position. Rollers may be constructed of metals, wood, or plastic and can be arranged in an optimum position, depending upon the articles to be carried.
To move loads on level or inclined paths, or declining paths that exceed the angle of sliding or rolling friction of the particular material to be conveyed, powered conveyors must be employed. There are various types of powered conveyors. Belt conveyors move loads on a level or inclined path by means of power-driven belts. Belt conveyors with rough-top belts make possible inclines up to 28°; cleated belts are limited on degree of incline only by the position of the center of gravity of the conveyed item.
Live-roller conveyors move objects over series of rollers by the application of power to all or some of the rollers. The power-transmitting medium is usually belting or chain.
Vibrating conveyors are designed to move bulk materials along a horizontal, or almost horizontal, path in a controlled system. They can be used to simply transport material from one point to another or to perform various functions en route, such as cooling, drying, blending, metering, spreading, and, by installing a screen, or dedusting.